In humans, the heart beats to sustain life. In normal operation, it pumps blood through the various parts of the body. More particularly, the various chamber of the heart contract and expand in a periodic and coordinated fashion, which causes the blood to be pumped regularly. More specifically, the right atrium sends deoxygenated blood into the right ventricle. The right ventricle pumps the blood to the lungs, where it becomes oxygenated, and from where it returns to the left atrium. The left atrium pumps the oxygenated blood to the left ventricle. The left ventricle, then, expels the blood, forcing it to circulate to the various parts of the body.
The heart chambers pump because of the heart's electrical control system. More particularly, the sinoatrial (SA) node generates an electrical impulse, which generates further electrical signals. These further signals cause the above-described contractions of the various chambers in the heart, in the correct sequence. The electrical pattern created by the sinoatrial (SA) node is called a sinus rhythm.
Sometimes, however, the electrical control system of the heart malfunctions, which can cause the heart to beat irregularly, or not at all. The cardiac rhythm is then generally called an arrhythmia. Arrhythmias may be caused by electrical activity from locations in the heart other than the SA node. Some types of arrhythmia may result in inadequate blood flow, thus reducing the amount of blood pumped to the various parts of the body. Some arrhythmias may even result in a Sudden Cardiac Arrest (SCA). In a SCA, the heart fails to pump blood effectively, and, if not treated, death can occur. In fact, it is estimated that SCA results in more than 250,000 deaths per year in the United States alone. Further, a SCA may result from a condition other than an arrhythmia.
One type of arrhythmia associated with SCA is known as Ventricular Fibrillation (VF). VF is a type of malfunction where the ventricles make rapid, uncoordinated movements, instead of the normal contractions. When that happens, the heart does not pump enough blood to deliver enough oxygen to the vital organs. The person's condition will deteriorate rapidly and, if not reversed in time, they will die soon, e.g. within ten minutes.
Ventricular Fibrillation can often be reversed using a life-saving device called a defibrillator. A defibrillator, if applied properly, can administer an electrical shock to the heart. The shock may terminate the VF, thus giving the heart the opportunity to resume pumping blood. If VF is not terminated, the shock may be repeated, often at escalating energies.
A challenge with defibrillation is that the electrical shock must be admitted very soon after onset of VF. There is not much time: the survival rate of persons suffering from VF decreases by about 10% for each minute the administration of a defibrillation shock is delayed. After about 10 minutes the rate of survival for SCA victims averages less than 2%.
The challenge of defibrillating early after the onset of VF is being met in a number of ways. First, for some people who are considered to be at a higher risk of VF or other heart arrhythmias, an Implantable Cardioverter Defibrillator (ICD) can be implanted surgically. An ICD can monitor the person's heart, and administer an electrical shock as needed. As such, an ICD reduces the need to have the higher-risk person be monitored constantly by medical personnel.
Regardless, VF can occur unpredictably, even to a person who is not considered at risk. As such, VF can be experienced by many people who lack the benefit of ICD therapy. When VF occurs to a person who does not have an ICD, they collapse, because blood flow has stopped. They should receive therapy quickly.
For a VF victim without an IUD, a different type of defibrillator can be used, which is called an external defibrillator. External defibrillators have been made portable, so they can be brought to a potential VF victim quickly enough to revive them.
During VF, the person's condition deteriorates, because the blood is not flowing to the brain, heart, lungs, and other organs. Blood flow must be restored, if resuscitation attempts are to be successful.
Cardiopulmonary Resuscitation (CPR) is one method of forcing blood flow in a person experiencing cardiac arrest. In addition, CPR is the primary recommended treatment for some patients with some kinds of non-VF cardiac arrest, such as asystole and pulseless electrical activity (PEA). CPR is a combination of techniques that include chest compressions to force blood circulation, and rescue breathing to force respiration.
Properly administered CPR provides oxygenated blood to critical organs of a person in cardiac arrest, thereby minimizing the deterioration that would otherwise occur. As such, CPR can be beneficial for persons experiencing VF, because it slows the deterioration that would otherwise occur while a defibrillator is being retrieved. Indeed, for patients with an extended down-time, survival rates are higher if CPR is administered prior to defibrillation.
Advanced medical devices can actually coach a rescuer who performs CPR. For example, a medical device can issue instructions, and even prompts, for the rescuer to perform CPR more effectively.
External defibrillators are used not only to treat victims of sudden cardiac arrest experiencing VF, but also for cardioversion of other tachyarrhythmias (such as atrial fibrillation) that may be experienced by a person not in cardiac arrest. For purposes of both defibrillation and cardioversion, most manual defibrillators provide a button or similar control for the rescuer to cause the device to administer an electric shock through the heart of the patient. For purposes of cardioversion, however, it is frequently important to precisely time the delivery of the shock relative to the patient's intrinsic heart rhythm, so that the shock does not inadvertently exacerbate the situation and cause the patient's condition to worsen, perhaps resulting in ventricular fibrillation. Such timing is extremely difficult or impossible for a defibrillator user to achieve based upon their own observation of the ECG rhythm and physical ability to push the shock button at the precise desired time.
Manual defibrillators, and some Automated External Defibrillators (AEDs), may therefore provide a sync (or synchronization) function that, upon activation, automatically adjusts the timing of shock delivery to be coincident (e.g., not simultaneous with the QRS complex, but with a fixed delay) with the next detected QRS complex or R wave in the monitored ECG signal in order to avoid delivering the shock during the T wave, which may result in the initiation of VF by stimulating during the vulnerable period. So, when this button is pushed, the defibrillator waits for the optimum instant (based upon the QRS complex or R wave of the monitored ECU signal) and then causes the shock to be delivered at that instant. This defibrillator operating mode is frequently called synchronized shock mode, or sync mode. When the defibrillator is not in this synchronized shock operating mode, it is in asynchronous shock mode.
While synchronized cardioversion is a common procedure, it is also fraught with user error. For example, the user may forget or ignore how the device is configured to behave after delivery of an initial synchronized shock. Also, even manual defibrillators that are configured to remain in sync mode or return to asynchronous mode after a synchronized discharge can create a hazard. For example, if the user is unaware or inattentive to the fact that the device is configured to return to asynchronous mode and the patient requires another synchronized shock, the user may inadvertently deliver an asynchronous shock, which could potentially trigger ventricular fibrillation. On the other hand, if the user is unaware or inattentive to the fact that the device is configured to remain in sync mode and the patient develops a pulseless tachyarrhythmia requiring an asynchronous shock, the user will not be able to deliver the necessary therapy until he or she recognizes that the device is still in sync mode and they manually deactivate the sync function.
In an automated device such as an AED or a wearable cardioverter/defibrillator, the device may only have an automatic mode in which it automatically determines what shocking mode to use (e.g., synchronous versus asynchronous) based on the monitored ECG signal. There would generally not be a default synchronization mode in the device configuration; instead, the synchronization mode would be based on the monitored ECG signal. Furthermore, in situations where the rhythm identified from the monitored ECG signal hovers between arrhythmias that require either synchronous or asynchronous shock, the device may have a programmable setting to configure a smart algorithm to either require at least 30 seconds of continuous detection before switching synchronization mode, or have a voting schema that uses multiple ECG segments.
For the wearable device, the shock recipient (e.g., patient) may be involved in the selection of the sync mode by providing input on his condition (e.g., by saying “I am awake” for synchronizing the shock).
A patient's ECG can be measured by a virtually infinite number of ECG electrode locations, also referred to herein as ECG leads. In clinical settings, it is common for a “12-lead” evaluation to be performed. It is also typical for the ECG from lead I or lead II to be used when setting a device in “sync mode.” There may be some situations in which the ECG in the preferred lead may have a QRS complex, or a specific feature such as an R-wave, amplitude that is lower than another portion of the cycle which should not be used for synchronizing a shock such as the T wave, for example. In such cases, synchronizing to a fiducial marker other than the QRS complex may cause a cardioversion shock to be applied during a non-desirable portion of the cardiac cycle.